www.studyguide.pk
Teaching A2 Chemistry Practical Skills
Appendix 2
19. The preparation and purification of methyl-3-nitrobenzoate
Student Sheet
In this experiment you will learn or develop skills in preparative organic chemistry by making
and purifying a sample of an aromatic nitro compound. You will assess the purity of your
product by measuring its melting point and comparing it with the data book value.
Intended lesson outcomes
By the end of this exercise you should be able to:
•
•
•
•
•
•
Determine the weight of a material using the weighing by difference method
Handle hazardous materials safely
Purify by recrystallisation
Determine the yield and melting point of a reaction product
Estimate the purity of a reaction product
Write a mechanism for the electrophilic substitution reaction involved in nitration
Background information
Nitration of an aromatic ring occurs by an electrophilic substitution reaction. The electrophile is
the nitronium ion, NO2+, which is prepared by the reaction between concentrated nitric acid and
concentrated sulphuric acid. The equation for this reaction is given below.
HNO3 + 2H2SO4 → NO2+ + 2HSO– + H3O+
The reaction conditions necessary for the nitration step which follows depend on the nature of
the molecule being nitrated; as does the position into which the nitro group will be substituted.
Using methylbenzoate, the reaction occurs readily, so that a relatively low temperature is
sufficient, and the nitro group tends to be substituted into the 3-position on the benzene ring.
Great care must be taken that appropriate cooling is used, and that the hazardous materials
used are safely handled. Purification of the crude product is by recrystallisation from the
minimum amount of hot ethanol.
The overall equation for the nitration reaction is
O
H3CO
C
+
H3CO
HNO3
O
NO2
C
+
H2O
Once the nitro group has been introduced into the ring, it may be readily reduced to an amine
group, by heating it with tin and concentrated hydrochloric acid. This reaction sequence
provides a very useful route to the synthesis of aryl amines.
Question
Write a mechanism for the electrophilic substitution reaction involved in this reaction, showing
clearly the structure of the intermediate species formed.
Safety
There are potentially hazardous substances involved in this exercise. You must follow all
health and safety instructions given to you by your teacher. Materials safety data sheets should
be consulted so that the correct action can be taken in event of a spillage and/or accident.
97
© University of Cambridge International Examinations 2006
www.xtremepapers.net
www.studyguide.pk
Teaching A2 Chemistry Practical Skills
Appendix 2
You must wear eye protection throughout this experiment
Methylbenzoate is harmful
Nitric acid and sulphuric acid are corrosive
Ethanol is highly flammable
Procedure
1.
Measure 9.0 cm3 of concentrated sulphuric acid into a dry 100 cm3 conical flask and place
the flask in an ice bath.
2.
Add about 4.0 cm3 of methylbenzoate to a small measuring cylinder. Weigh the measuring
cylinder and methylbenzoate recording the mass.
3.
Prepare a nitrating mixture by carefully mixing 3.0 cm3 of concentrated nitric acid and 3.0
cm3 concentrated sulphuric acid in a test tube. Place this test tube in the ice bath.
4.
Add the methylbenzoate to the conical flask; swirling the flask while you do so. Weigh the
emptied measuring cylinder and calculate the mass of methylbenzoate added. This
process is known as weighing by difference.
5.
Using a dropping pipette, add the nitrating mixture to the conical flask. You must swirl the
flask after each addition and, by monitoring the thermometer, adjust the rate of addition to
ensure that the temperature does not exceed 15 °C. Do not rush the addition; it should
take about 10–15 minutes.
6.
After the addition is finished, allow the flask to stand at room temperature for about 10
minutes. Pour the reaction mixture from the flask into a small beaker containing about 40–
50 g of crushed ice. Stir until the product solidifies.
7.
Filter off the impure solid product, preferably using vacuum filtration, and wash the product,
first with cold water and then with about 5 cm3 of ice-cold ethanol. Allow the solid to dry as
much as possible.
Recrystallisation
8.
Place the impure product in a clean 100 cm3 conical flask and add 10 cm3 ethanol. Gently
warm the mixture in a water bath. If some of the solid fails to dissolve add further 2 cm3
portions of ethanol until it does dissolve, but do not continue adding ethanol if it is clear
that some material is not going to dissolve; your aim is to produce as concentrated a
solution as possible.
9.
If necessary, filter the solution using a filter funnel or Buchner funnel to remove any
insoluble material. To prevent crystals forming during filtration, pre-heat the funnel by
adding hot solvent to the filter paper in the funnel; allow time for the solvent to run through.
Discard this hot solvent before you use the hot funnel to filter your solution.
10. Cool the filtrate in an ice bath until crystals form.
11. Filter off the crystals, preferably by vacuum filtration, and allow them to dry as much as
possible. Transfer the crystals to a clean piece of filter paper on a watch glass. Let the
crystals stand overnight to dry out completely.
98
© University of Cambridge International Examinations 2006
www.xtremepapers.net
www.studyguide.pk
Teaching A2 Chemistry Practical Skills
Appendix 2
12. Weigh the crystals by adding them to a pre-weighed sample bottle and reweighing.
13. Using the melting point apparatus provided, determine the melting temperature of your
crystals.
14. Transfer your crystals to a sample bottle labelled with your name.
Theoretical maximum mass of product
15. From the mass of methylbenzoate added, deduce the maximum mass of methyl-3nitrobenzoate which could be produced (i.e. if all the methylbenzoate had been converted
into product).
Percentage yield of product
16. Calculate the percentage yield of your preparation. You do this by comparing your actual
yield of product with the theoretical maximum yield and expressing the result as a
percentage.
Purity
17. The pure product would have a melting point of 77 °C ±1 °C; compare your melting point to
this value. The further away, and the wider the melting range, your melting point is
compared to 77 °C the less pure is your product. Also, your crystals should be dry and pale
yellow in colour. Comment on the appearance, yield and purity of your product and then
show your product to your teacher, who will also assess its appearance and yield.
99
© University of Cambridge International Examinations 2006
www.xtremepapers.net
www.studyguide.pk
Teaching A2 Chemistry Practical Skills
Appendix 2
19. The preparation and purification of methyl-3-nitrobenzoate
Teachers’ Notes
Overall, the exercise provides a range of learning experiences for the student, which include:
weighing and measuring, the safe handling of materials, temperature control, recrystallisation
techniques and melting point determination.
Intended learning outcomes
Please see the Student Sheet
A suggested approach
The exercise provides considerable opportunity for students to practice and develop their
manipulative skill. It may also help them to appreciate that organic reactions, unlike many
inorganic reactions, require care and patience if they are to be successful, and that, despite all
their care, yields are likely to be limited.
It would be worth spending a little time discussing the usefulness of this reaction in the
synthesis of aryl amine and their products.
This exercise involves the use of potentially hazardous materials and so students should be
closely supervised, unless they have considerable experience of organic preparations. The
nitration reaction proceeds readily and so, in order to prevent multiple substitution, it is
necessary to cool the reaction mixture. The use of vacuum filtration, using a Buchner filtration
system, is preferred. The exercise may be successfully completed using gravity filtration,
however, but it does take longer; especially in the final drying stage.
Note
The product from this reaction could be reduced to make the aryl amine. This is not advised,
however, as the purity of the nitrated product cannot be guaranteed. If your students were to
perform such a reduction reaction, it would be better to use a commercial sample of an
appropriate aromatic nitro compound. The reaction process involves the use of concentrated
hydrochloric acid, concentrated sodium hydroxide, ether extraction and steam distillation. The
final purification of the phenylamine product is by distillation. This reaction sequence requires
considerable practical experience but could, if wished, be performed as a demonstration.
Details may be found in:
Advanced Practical Chemistry
by JS Clarke & S Clynes (English University Press)
Technical information
Requirements per student/group
Apparatus
•
•
•
•
•
•
•
•
•
Two 100 cm3 conical flasks
Large beaker/small trough/small washing up bowl for ice bath
100 cm3 beaker for crushed ice
Beaker for water bath
Two/three 10 cm3 measuring cylinders
Thermometer for monitoring temperature during nitration
Stirrer – unless the thermometer may be used for this purpose
Buchner funnel and flask or filter funnel
Filter paper for funnel and for drying crystals
100
© University of Cambridge International Examinations 2006
www.xtremepapers.net
www.studyguide.pk
Teaching A2 Chemistry Practical Skills
Appendix 2
•
•
•
•
•
Suction pump (if Buchner system used)
Two dropping pipettes
The apparatus normally used for melting point determinations
Marker pen for labelling test tubes
Access to balance
Materials
•
•
•
•
•
•
•
Access to methylbenzoate
Access to concentrated nitric acid
Access to concentrated sulphuric acid
A supply of ethanol
A supply of crushed ice
Distilled/deionised water
A supply of hot water for the recrystallisation
Safety
The main points are included on the Student Sheet. However:
It is essential that a risk assessment is carried out before a decision is taken to go ahead with
this exercise. MSDS sheets should be consulted so that the correct action can be taken in
event of a spillage and/or accident.
•
•
It must be made clear to students that potentially hazardous materials are in use and that
precautions are needed to minimise the risk to themselves and to others.
Your MUST be prepared to intervene if a student seems to be unsure of a procedure, or is
performing an unsafe operation.
A number of ways in which the exercise may be made less hazardous and more likely to
succeed include:
(i) Measuring out the concentrated acids yourself and supplying them to the students in a
labelled conical flask labelled A and a labelled test tube.
(ii) Using hot water from a kettle for the water bath during the recrystallisation stage.
(iii) Supplying small pieces of crushed ice for the ice baths.
(iv) Telling the students that their safe working practices are being assessed; whether this
is true or not!
101
© University of Cambridge International Examinations 2006
www.xtremepapers.net